The overall goal of this project is to understand the mechanisms of seasonal rhythms in reproduction and development of mammals. Specifically, I propose to determine the mechanisms by which the mother's photoperiodic history influences the development of her offspring, and to describe the development of social behaviors underlying seasonal variations in fertility. The model species, the meadow vole (Microtus pennsylvanicus), was chosen because there is extensive knowledge of its natural history upon which to base hypotheses for experimental laboratory analysis. A growing body of research indicates that dams communicate photoperiod information to their fetuses and this information influences neonatal development. Meadow vole dams are capable of fetuses and this information to their offspring pre- and postnatally. Additionally, the length of time the mother has been in short days (whether she is still sensitive to short daylengths or has become photorefractory) prior to impregnation influences development of her offspring pre-and postnatally. Communication of maternal photoperiodic history to the offspring presumably allows them to develop appropriately for the season of their birth. This project addresses the mechanism(s) of the maternal photoperiodic information transfer and the influence of photoperiod history on development of social behaviors. Specific aims include: 1) Delineation of the control of maternal-litter contract time as a function of pup thermoregulatory abilities which are influenced by the maternal photoperiodic history. 2) Determining the role of melatonin in communicating information about photoperiodic history between the dam and her offspring by measurement, removal and replacement experiments. 3) Determining the effect of maternal photoperiodic history, current photoperiod, and age of partner on mating behavior and afflictive/aggressive social interactions that control population fertility. Successful completion of the proposed projects will enhanced our understanding of mechanisms in the mothers and pups that result in significant seasonal differences in physiological and social development. A growing body of data demonstrates that humans are also affected by the changing seasons both neonatally and as adults. The extrapolation of data from meadow voles to other short-lived rodents is plausible, and the possibility of similar mechanisms operating in humans must be entertained. Ultimately seasonal effects on development, directed by the human mother, may prove to be important in conjunction with other environmental factors such as adequacy of nutrition.